Customary International Humanitarian Law refers to a body of law which has emerged, not from written treaties and agreements, but from the acts of states. These rules develop over time as states conduct themselves in a particular and consistent manner, and in the belief that they are legally required to do so.

There is a series of rules which may be considered customary international humanitarian law. This means that even if a State has not ratified a particular IHL treaty, it is nevertheless legally bound by the rule.

In the early 1990s, at the request of States, the ICRC's assistance was sought in identifying the unwritten rules of customary international humanitarian law. In 2005, the ICRC published its study on which rules of humanitarian law reflect widespread and uniform state practice such that they may be considered part of customary international law.



Useful links:

More information on treaties and customary international humanitarian law
More information on the report published by the ICRC regarding customary international humanitarian law